Feedback system to automatically couple microwave energy into an applicator

ABSTRACT

A microwave processing system comprising a microwave applicator and a microwave source. A microwave source generates a microwave field at an output frequency. A frequency of the microwave field in the elongated chamber and the output frequency of the microwave source are matched through the use of controlled feedback of critical material and process parameters.

This application claims the priority from U.S. Provisional ApplicationNo. 60/034,717 filed Jan. 6, 1997.

FIELD OF THE INVENTION

The present invention relates to microwave applicators and, moreparticularly, to a system for matching an input frequency with theresonant frequency of a loaded applicator.

BACKGROUND OF THE INVENTION

Microwave radiation can be applied to a material in a number of ways,using single mode, multimode applicators, traveling wave applicators,slow wave applicators, fringing field applicators and through freespace. Each of the aforementioned methods of coupling microwave energyinto a material has its advantages and disadvantages which usuallydepend on the dielectric properties, size and shape, of the materials tobe heated and the type of processing (batch, continuous, . . . etc.) tobe performed.

Efficient microwave energy transfer is a function of many variables asprocessing occurs. A number of these variables are material related,e.g., the material type and density and material temperature as well asthe time history of both the material temperature and the appliedelectric field. As the material is heated, the dielectric constant mayexhibit hysteresis in temperature and electric field strength. Dependingon the nature of the change of the dielectric constant, this may resultin the application of a non-uniform electric field or thermal runaway,e.g., hot and cold spots within the material or a mismatch between theresonant frequency of the loaded applicator and the microwave energysource. As the dielectric properties of the load change, the propertiesof the loaded applicator also change, for example, the resonantfrequency of the applicator may change.

Other factors that influence coupling are related to the applicator,material geometry and size and the frequency or wavelength of theelectromagnetic energy. Electromagnetic coupling depends on applicatorsize and geometry, material size and shape, the position of the materialwithin the applicator, and even the relative sizes and shapes of thematerial and the applicator. In addition, both the applicator andmaterial dimensions may change during heating which further complicatesthe efficient transfer of energy to the material.

Accordingly, a problem arises when attempting to maintain efficientcoupling of the microwave energy into the applicator. This is especiallydifficult with a single mode, high Q factor applicator. This type ofapplicator can be tuned to specific electric field patterns (resonancemodes) by varying the volume of the applicator. This, however, islaborious and provides difficult process control.

One such approach is found in U.S. Pat. Nos. 4,507,588, 4,585,668,4,630,566, 4,727,293, and 4,792,772 (Asmussen), all of which disclosemethods and apparatuses in which a single mode resonant microwaveapplicator can be critically coupled by varying two separate, almostorthogonal variables, specifically the cavity length (by moving a shortcircuit) and the antenna position.

The Asmussen devices include a variable penetration antenna structurewhich acts to launch radiation into the applicator. The main advantageof the Asmussen device is that it enables complete critical couplingover a wide range of impedance's (generated by the load in theapplicator) and without the use of any external coupling structure.Critical coupling can thus be achieved by moving the short and theantenna appropriately.

By moving the flat part of the cavity wall (in a cylinder) in thez-direction (e.g., along the centerline of the cylinder), a wide rangeof electromagnetic modes can be established and maintained, even as theload varies (due to processing, e.g., temperature changing, materialcuring, etc.) As a result, if the load changes during processing (e.g.,the dielectric properties change, due to increased temperature, curing,phase change in the material and so forth), the resonant frequency inthe cavity changes from an initial, fixed processing frequency, usually2450 MHz or 915 MHz (which are the ISM bands allowed by the FederalCommunication Commission (FCC)). It should be understood that the outputfrequency of a magnetron, by far the most common microwave generatingdevice available, is relatively narrow and is a fixed frequency (becausethe device itself is a resonant device) and cannot be convenientlyvaried over a wide range of frequencies dynamically.

U.S. Pat. No. 5,471,037 (Goethal) discloses a single mode cylindricalapplicator that operates in the TM_(02n) resonant mode. The microwaveapplicator is designed to process monomers in order to produceprepolymers. The size of the microwave applicator is selected accordingto the particular monomers being processed (e.g., fixed dimensionapplicator). Therefore, there is no mechanism for altering the diameterof the applicator to account for substantially different loads orsubstantially different dielectric properties.

U.S. Pat. No. 3,461,261 (Lewis) relates to a TM_(02n) applicator thatprocesses threads and yarns with the workpieces passing along thecentral axis of the applicator. The dimensions of the microwaveapplicator are selected according to the materials being processed(e.g., fixed dimension applicator).

The electric field pattern sustained by the TM_(0y0) series of modes,where y=1, 2 or greater, is oriented along the z-axis of the applicatorand is of constant intensity along the entire length of the applicatorfor an empty cavity. This is an ideal mode for the processing of wideweb-like materials. Referring to FIG. 1 (a mode chart), it can be seenthat the TM₀₁₀ mode is independent of the cavity length. Therefore, alow loss, infinitely long applicator is capable of sustaining the sameelectric field intensity throughout the length.

In all of the aforementioned prior art, all of the available methodsrely upon a mechanical change of dimension of the applicator to matchthe resonant frequency of the applicator with the output frequency ofthe microwave source. This can be accomplished through the use ofstepper motors and sometimes complex, computer driven feedback loops andare slow because of the time required for a motor to complete motionbefore another iterative measurement of the degree of tune is taken andthe tune further improved.

Accordingly, an object of the present invention is to provide a means tomaintain maximum coupling efficiency of microwave energy to a microwaveapplicator through the use of controlled feedback of critical materialand process parameters.

It is a further object of the invention to provide a microwaveapplicator whereby field interactions with the material being processedare controlled through matching of the output frequency of the microwavesource to the loaded applicator.

Another object of the invention is to provide a means for only allowingthe desired mode and therefore electric field pattern within a loadedmicrowave applicator automatically through the use of a controlledfeedback of critical material and process parameters.

SUMMARY OF THE INVENTION

A means to automatically maintain maximum coupling efficiency of amicrowave source to a loaded microwave applicator automatically bycontrolled feedback. The loaded microwave applicator itself acts as afilter to provide a signal to the microwave source, forcing the sourceto match the resonant frequency of the loaded applicator. A frequency ofa microwave source and loaded microwave applicator are thereby matchedthrough the use of controlled feedback of critical material and processparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a microwave applicator in accordance with the presentinvention.

FIG. 2 illustrates a conventional block diagram for a high powermicrowave circuit.

FIG. 3 illustrates a block diagram of a first embodiment of themicrowave applicator in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a microwave applicator 10 in accordance with thepresent invention includes an elongated chamber 12 with a means forcoupling microwave energy, either through a side launch as shown or anend launch structure and a means for introducing a workpiece to beprocessed. FIG. 1 shows a schematic specifically designed to process acontinuous, wide sheet-like material using the TM010 mode (a lengthindependent mode, ie without a node across the width of the applicator).Conventionally, the applicator would have a means for varying thelength, through a movable short, or the width by varying the diameter ofthe applicator, as shown in co-pending application U.S. ProvisionalApplication No. 60/034,717 filed Jan. 6, 1997.

A waveguide 14, coupled to elongated cylinder chamber 12, propagates amicrowave field into elongated chamber 12. The waveguide 14 can beconnected to a waveguide to coaxial adapter, which allows a coaxialcable to transmit the microwave power from the microwave source to theapplicator.

In order to control and maintain microwave field uniformity duringmaterial processing and to overcome the difficulty in changing effectivecavity dimensions, the present invention provides a novel configurationfor tuning the resonant frequency of microwave applicator 10 through theuse of controlled feedback of critical material and process parameters.The various preferred embodiments will be discussed in detail below.

Referring to FIG. 2, it is normally desirable to provide minimal reversecoupling of the microwave applicator 10 to the power source 20 sinceexcess reflected microwave energy results in unstable operation of thepower source and can result in overheating of the source 20 and hencereduced lifetime. Hence a circulator 22 with 3 ports (24, 26 and 28) anddummy load 30 is conventionally used. Microwave energy passes freelyfrom port 1 (24) to port 2 (26) in the forward direction and freely fromport 2 (26) to port 3 (28) in the reverse direction, resulting in thereflected microwave power being transferred and dissipated in the dummyload 30. Only a poor impedance match between the dummy load and 28 willresult in any radiation being transferred to port 1 (24) from port 3(28).

Referring to FIG. 3, a first embodiment of the present inventionincludes a microwave source 20 preferably having a broad bandwidth whichis automatically tuned to the frequency of microwave applicator 10through the use of controlled feedback of critical material and processparameters from the applicator. In particular, critical material andprocess parameters are fed back to microwave source 20 by utilizing theapplicator as a band-pass filter and sampling the radiation from withinthe applicator and directing the radiation towards the broadband source20. This has the effect of using the resonance of the applicator as thesource of radiation to the broadband source 20, which will then matchthe output frequency of the broadband source to the resonance ofmicrowave applicator 10.

In the event that the resonant frequency of microwave applicator 10moves (due to a change in the dielectric properties of the load due tochanging chemistry, temperature, change in composition, etc), thefrequency of the radiation being sampled within the microwave applicator10 will therefore change and hence the amplified signal from themicrowave source 20 will change to match the properties of theapplicator.

In this embodiment, the transmitted radiation is utilized as a frequencylock-in signal for microwave source 20. This method is very fast, with atime scale of microseconds, but relies upon microwave source 20 havingsufficient bandwidth and the exclusion of other resonant frequencies inthe range onto which microwave source 20 can lock.

There are currently a number of methods for generating high powermicrowave energy, such as using a magnetron, klystron, gyrotron,traveling wave tube (TWT) and solid state amplification. Of theaforementioned microwave sources, by far the lowest cost method ofgenerating microwave energy is the cavity magnetron (which is found inall home microwave ovens). Although cavity magnetrons are inexpensive,they have a very narrow output spectrum (bandwidth) and can only bepulled or moved about 10 to 15 MHz at 2.45 GHz. Other types ofmagnetrons, including coaxial types, have a much greater range overwhich the output frequency can be pulled, but tend to be more expensiveand limited in power output. Gyrotrons and klystrons also operate over avery narrow bandwidth. In this embodiment, it is preferred thatmicrowave source 20 is a broadband source, such as a TWT, or a solidstate amplifier.

FIG. 3 illustrates a first embodiment of microwave applicator 10 whichemploys a frequency locking structure to pull the frequency of themicrowave source to that of the applicator. To accomplish the foregoing,a plurality of very broad band microwave supplies 40, such as TWTamplifiers or solid state amplifiers, are utilized to match asubstantially wider range of resonant frequency loads, ranging fromapproximately 100 MHz to approximately 300 MHz. It is, however,preferable that microwave supplies 40 have a bandwidth betweenapproximately 5-50% of the output frequency. A probe 46, positioned inelongated chamber 12 of microwave applicator 10, transmits a signalcontaining critical material and process parameters of elongated chamber12 back to microwave supplies 40, across a gain control module 42 andphase control module 44. The judicious positioning of probe 46 canresult in the elimination of certain modes, since there will be nocoupling of the electric field from that point in the elongated chamberback to the amplifier--hence spurious modes occurring at a frequencydifferent to the desired mode can be eliminated.

As can be seen, this is accomplished by feeding some of the microwaveradiation (i.e., the signal) in elongated chamber 12 initially to gaincontrol module 42 and phase control module 44, via probe 46. Gaincontrol module 42 monitors the amplitude of the microwave radiation andensures that the microwave input of the microwave supplies 40 is withina predetermined amplitude range so that there is sufficient signalstrength to be amplified, but not too much power which would result inthe destruction of the power supply 40. Thereafter, phase control module44 adjusts or tunes the input phase of microwave supplies 40 to preventbeating of the power supply and resulting in variable input power to theelongated chamber 12. Typically, the phase control is fixed in theinitial setup of the feedback loop and does not require adjustmentthereafter.

In the preferred embodiment, a broadband solid state amplifier is usedto amplify the signal samples from the microwave applicator. Initially,a broadband frequency source or noise is fed into the microwaveapplicator. The resonant frequency of the loaded applicator is thensampled by a probe inserted into the microwave applicator and the signalfrom that probe fed to the input stage of the amplifier.

There are a number of challenges to accomplishing this since theelectric field strength in the microwave applicator will increasesubstantially from the initial state in which minimal microwave power issampled from the applicator to full power operation in which the powerfed back may drive the amplifier into saturation or destroy the inputstage of the amplifier. However, if the signal being input to theamplifier is too small, signal strength noise may dominate and thefeedback effect is negated. The role of the gain control module, 42, isto provide a stable input power level to the amplifier.

To overcome this the potential of saturation of the input signal, apower limiter can be used. Power limiters are typically employed inreceivers to protect the front end of these units from excess power.There are several kinds of limiter, however, the most common kind usesdiodes to limit the signal. Diode limiters have good high powerprotection capabilities but can, but can have distortion problems. Diodelimiters can not provide input boost for low power signals going intothe amplifier.

A unity gain amplifier is simply an amplifier that is designed toproduce the same amount of output power regardless of the input signalstrength. The disadvantage of these devices is that they are sensitiveto power surges and can be destroyed easily. However, due to therelatively low cost of these devices compared to the high poweramplifiers used in the next stage, it is more desirable that the unitygain device be destroyed accidentally than the power amplifier.

A variable attenuator provides a similar function as the diode limiterand provides less distortion, but is more complex. In this type ofdevice, the input signal is attenuated by an amount that increases withpower coming into the attenuator, thus providing a constant outputpower.

For the first, second and third embodiments, microwave radiation can becoupled into microwave applicator 10 using an iris, loop or antennaplaced on the outer surface of elongated chamber 12 or from the end ofelongated chamber 12 (e.g., end launch). The choice between theaforementioned devices depends on the material to be processed, themanner in which the dielectric properties change during processing andthe resonant mode being utilized. Multiple launchers can also be used toimprove uniformity and to generate higher power levels in the cavity.They allow the resonant cavity of microwave applicator 10 to act as apower combiner and, at the same time, can be used to minimize anynon-uniformities.

In summary, the present invention provides a system in which the outputfrom a microwave applicator automatically tunes the output frequency ofthe microwave source to that of the applicator for varying loads byutilizing controlled feedback of critical material and processparameters. As can be appreciated, the resonant frequency in themicrowave applicator can be maintained without varying the structuraldimensions of the applicator.

The invention having thus been described with particular reference tothe preferred forms thereof, it will be obvious that various changes andmodifications may be made therein without departing from the spirit andscope of the invention as defined in the appended claims.

It is usual that a particular electric field pattern is desired for aparticular load. For example, for a flat surface to be processed, aTE₁₁₁ mode is desirable, for a tubular structure, a TM_(02x) mode ismore desirable and for a wide web of material, a TM₀₁₀ lengthindependent mode is desirable. Each of these modes can be excited in aparticular microwave applicator by providing the correct launchstructure. If the applicator is cylindrical in shape, the TE₁₁₁ mode canbe excited by a straight antenna or a loop antenna mounted in the sideof the cylinder at a maximum field region, while a waveguide launchplaced in the same position would not be able to excite that mode.Similarly, by placing the antenna at a null point in the electric fieldof the mode would result in the mode not being able to be excited.Hence, the judicious placement of a launch structure, certain modes canbe prevented from being launched and sustained in the applicator. Thisknowledge can be used to prevent the excitation of certain unwantedmodes.

However, modes such as the TE₁₁₁ mode have a number of higher orderanalogous modes, TE₁₁₂, TE₁₁₃ for example, in which the electric fieldpattern repeats in the z-direction. These modes occur at higherfrequencies than the base mode. Unfortunately, even through good designand placement of a launch structure, more than one mode can besimultaneously excited if the bandwidth of the power amplifier was greatenough. With the use of conventional magnetron power supplies this isless problematic due to the very narrow bandwidth of those devices. Inthe case where a very wide bandwidth power amplifier were to be utilizedwith a feedback system as is the object of this invention, it isimportant to be able to exclude power from going to these modes andtherefore maintain a pure electric field pattern within the applicatorand therefore in the material being processed.

In the present invention, a signal is being sampled from the microwaveapplicator to be used as the input to the power amplifier. With the caseof a broadband power amplifier supplying an applicator in which a numberof modes exist simultaneously, a number of different frequencies will befed back to the power amplifier. The power amplifier will then amplifyall of these frequencies by the same amount with the relative outputpower of each frequency directly dependent on the power level on theinput side of the amplifier and gain of the power amplifier for eachfrequency (since most amplifiers are not completely linear withfrequency), thereby providing a frequency which will be fed back to theamplifier, and so on.

Since it is not desirable to have these unwanted modes, it is importantto exclude portions of the output spectrum of the power amplifier frombeing fed back to the amplifier. This can be accomplished in a number ofways: (i) judicious placement and design of sampling antennas; (ii)bandpass filters between the sample port and the power amplifier; (iii)electronic, dynamic control over output spectrum of the power amplifier(specifically to limit the frequency range of the power amplifier).

In a similar fashion to being able to prevent the launch of certainmodes by the placement and type of the launch structure, it is alsopossible to limit the modes being sampled in the applicator. Forexample, if a probe antenna was inserted into the side of a cylindricalapplicator and a single TM mode was present, the antenna would not beable to pick up the field and hence it would appear that no mode waspresent. If a loop antenna was placed in the same position, a fieldwould be able to be detected. Similarly, if the loop antenna was placedat a position corresponding to a null in the field, no mode would bedetected because there is no coupling to the probe. Therefore, by thejudicious design and placement of a coupling probe, the types of modesthat can be sampled can be limited. By limiting the number of possibleresonant frequencies being sampled by the sampling probe, the number ofspurious modes being feedback to the amplifier can also be restricted.It is important to note that through the wrong design of placement ofthe sampling probe, the desired mode may not be obtained either.

By using more than one probe, two or more signals can be sampledsimultaneously and fed to the amplifier. In this case, a gain controlamplifier would be used after each probe to ensure the correct level ofsignal is set before recombination of input signals before the amplifierfor amplification into the microwave applicator. Such an arrangement mayprovide more uniform processing over large sample sizes.

Bandpass filters are relatively straightforward devices that are wellknown in the electronics industry. Normally, they are designed forrelatively low power levels and are relatively inexpensive. A bandpassfilter could be used either before the power amplifier (and thereforerestrict the range of frequencies (preferably one) being allowed to goto the amplifier) or after the amplifier and limit the frequencies(preferably to one) going to the applicator. There are severaldisadvantages of the second approach (i.e. placing the bandpass filterafter the power amplifier): (i) The amplifier would not be asefficiently amplifying the frequency desired; (ii) the filter would haveto be able to cool the energy being dissipated at the unwantedfrequencies; (iii) a high power capability is expensive. By placing thedevice before the power amplifier, only the desired frequency (orfrequencies) are allowed to be transmitted to the amplifier.

In the case where it is desired to launch two or more differentfrequencies in to the microwave applicator simultaneously, the signalfrom the applicator is split (if one probe can sample two modes) and abandpass filter adjusted to each of the desired frequencies is placedbefore the amplifier on each of the split cables. The signals are thencombined from each of the split cables at the amplifier. The gain isindividually adjusted to provide the desired level of power at eachmode.

All of the previous examples site direct feed back techniques. Indirectfeedback techniques are also possible. In this technique a signal otherthan primary driving signal is used.

One example of this is to feed back the output signal from the cavity toa discriminator. The discriminator is a circuit device that convertsfrequency into voltage and is commonly used in communication devices.This voltage is then used to drive a voltage controlled oscillator(VCO). The VCO's output can then be used to drive the power amplifierdriving the cavity. The VCO has the advantage that it provides constantpower output and a single frequency with no harmonics. Further, thefrequency range can be limited by controlling the voltage between thediscriminator and the VCO.

Another example of a indirect feedback is to use a higher harmonic ofthe resonant cavity as the feedback signal. The principle advantage ofthis is that these harmonics have much less power than the signal at theprimary driving signal making the higher order harmonics easier tofilter and control. For example if the 2nd harmonic of the TM₀₁₀ mode isused (the resonant TM₀₂₀ mode) the signal can then be feed through a 2xdivider (a common communication circuit) and feed into the drivingamplifier.

Another example of a indirect feedback is to mix the frequency down infrequency, filter and condition it, then mix it back up to drive thepower amplifier. Typically in communication circuits this is calledhedrodyning and the circuitry for doing it is common and wellunderstood. For example, the output of the cavity would be feed into acircuit called a mixer with another signal typically an I.F.(intermediate frequency). The mixer produces the multiplication of the 2signals. If the IF frequency is carefully selected part of this productis much lower in frequency (exactly dependent and controlled by the IFfrequency) and can be separated for the other components by bandpassfiltering. This lower frequency signal can then filtered and conditionedusing conventional rf technology including devices such as programmablesignal conditioners that are now available. The signal could also bedigitized and processed in a computer. Finally the conditioned andfiltered signal is mixed back up to the desired range of the poweramplifier and used to control the cavity.

What is claimed is:
 1. A microwave system for automatically providingsubstantially complete coupling of microwave energy to a microwaveapplicator comprising:a chamber containing a material load to be heatedwith microwave energy with a characteristic resonant frequency; a sourceto generate microwave energy suitable for being supplied to saidchamber; means for sampling said microwave energy from said chamber;means for transmitting said sampled microwave energy to a microwaveamplifier device; means for transmitting amplified energy to saidchamber.
 2. A microwave system as defined in claim 1, wherein saidsampling means is designed and placed to sample only microwave energyfrom a predetermined electromagnetic mode present in said chamber,thereby sampling only the frequency of the mode at the exclusion of allother frequencies.
 3. A microwave system as recited in claim 1 where thechamber sustains multiple electromagnetic modes simultaneously.
 4. Amicrowave system as recited in claim 1 where chamber sustains a singleelectromagnetic mode.
 5. A microwave system as recited in claim 1wherein said source to generate microwave energy is selected from thegroup consisting of a solid state amplifier and tube amplifier.
 6. Amicrowave system as recited in claim 5 in which the tube amplifer isselected from the group consisting of traveling wave tube amplifier,klystron and gyrotron.
 7. A microwave system as recited in claim 1 wheregain control is used to control said sampled microwave energy to saidmicrowave amplifier device at a variable predetermined level.
 8. Amicrowave system as recited in claim 7 where said gain control isselected from at least one of unity gain amplifier, bandpass filters anddiode limiters.
 9. A microwave system as cited in claim 7 in which thegain control is comprised of a voltage controlled oscillator driven by afrequency to voltage discriminator.
 10. A microwave system as recited inclaim 5 wherein said amplifier device emits an output signal rangingbetween 100 MHz and 300 MHz.
 11. A microwave system as defined in claim1, further comprising probe means, positioned in said chamber, forsampling said material load and transmitting a signal containingmaterial load and process parameters of said chamber to controllingmeans wherein said controlling means in response to said material loadand said process parameters, causes said microwave energy to match saidcharacteristic resonant frequency.
 12. A microwave system as recited inclaim 1, wherein said microwave amplifier device is directly coupled tosaid chamber.
 13. A microwave system as recited in claim 1, wherein saidmicrowave amplifier device includes amplifier means having a largebandwidth.
 14. A microwave system as recited in claim 1, furthercomprising means for controlling said characteristic resonant frequencyof said chamber.
 15. A microwave system as recited in claim 1, whereinsaid characteristic resonant frequency ranges between approximately 100MHz and approximately 300 GHz.